To meet the ever-increasing demand on communication capacity, optical transmission systems are moving towards high channel data rate (e.g., Terabit/s/channel) and high spectral efficiency (SE). Multi-carrier modulation with carriers arranged under the orthogonal frequency-division multiplexing (OFDM) condition is a promising technique to support high data-rate transmission with high SE. To satisfy the OFDM condition, the carriers have to be frequency locked. There is thus a need for generating a frequency-locked optical comb source (FLOCS). It has been proposed to generate a FLOCS by using a laser followed by two optical modulators.